Preheat your oven to 375°F (190°C).
Grease a 9×13-inch baking dish with butter.
In a small saucepan, combine the heavy cream, minced garlic, salt, pepper, and nutmeg. Heat over medium heat until just simmering, then remove from heat.
Layer half of the sliced sweet potatoes in the prepared baking dish.
Pour half of the cream mixture over the sweet potatoes.
Sprinkle half of the cheddar cheese and half of the Parmesan cheese over the top.
Repeat the layers with the remaining sweet potatoes, cream mixture, and cheeses.
Dot the top with small pieces of butter.
Cover the dish with aluminum foil and bake in the preheated oven for 30 minutes.
Remove the foil and bake for an additional 20-25 minutes, or until the top is golden brown and the sweet potatoes are tender.
Let the gratin cool for a few minutes before garnishing with fresh parsley and serving.
Variations & Tips

For a different flavor profile, try using Gruyère cheese instead of cheddar. You can also add a layer of caramelized onions between the sweet potato layers for added sweetness and depth. If you prefer a bit of heat, sprinkle some red pepper flakes into the cream mixture. For a lighter version, substitute half of the heavy cream with milk or use a lower-fat cheese.